At present, various illumination light sources such as an incandescent lamp, a fluorescent lamp and so on are used around us. Recently, a white LED, having a combination of characteristics such as low power consumption, long life, safety and so on, gets a lot of attention as an alternative illumination to the incandescent lamp or the fluorescent lamp. Further, a phosphor used in the white LED is also required to have higher performance regarding the emission efficiency and durability.
White LEDs are roughly classified into two types in terms of a method for producing a white color, and among them, the two-wavelength white LED that is the mainstream at present obtains a pseudo white color by using both an GaInN-based blue LED and a yellow phosphor YAG:Ce3+(Y3Al5O12:Ce3+) but is not excellent in color rendering property, and thus cannot emit white light close to the natural light (see Non-Patent Document 1).
On the other hand, there exists a three-wavelength white LED excitable with near-ultraviolet light or blue light and emitting white light closer to the natural light. For example, many silicate-based phosphors excite with near-ultraviolet light or blue light and emit visible light and are thus regarded as promising three-wavelength white LED phosphors, but none of them has a high luminance at a practical level (see Non-Patent Documents 2, 3).